One-third of the estimated 12 million new cases of sexually transmitted infections in the United States each year is attributable to Chlamydia trachomatis serovars D-K. In addition to the adverse reproductive consequences, such as pelvic inflammatory disease, ectopic pregnancy and infertility, untreated genital chlamydial infections also place women at a higher risk for acquiring the human immunodeficiency virus. Because chlamydiae are slow-growing, obligate intracellular pathogens, very little is known about the composition of surface or envelope components. Such molecules are primary candidates for vaccine design in other bacteria. Therefore, the purpose of these studies is to generate new information on chlamydial surface molecules which participate in key steps of the infectious process. In Specific Aims 1 and 2, the investigator will examine and characterize chlamydial envelope proteins, which are regulated by the availability of iron. Iron-regulated envelope proteins in other bacteria (i) are expressed in vivo during a natural infection, (ii) may elicit protective antibodies, and (iii) may serve an important role in host or tissue specificity. In Specific Aim 3, the PI will purify and generate antiserum against the chlamydial macromolecular surface structures called "projections." The overall impact of these studies should (i) advance our understanding of the cellular microbiology of chlamydial infection, and (ii) provide much needed information on chlamydial surface components with immunotherapeutic potential.